Mastering JavaScript Promises

The states and return values of a promise

From Chapter 1, Promises.js, we are already aware that a promise is based on three states. Let's brush up our memory on these states, in accordance with promises paradigm.

Promise has three states:

• Unfulfilled promise
• Fulfilled promise
• Failed promise

A promise exists within these three states.

The beginning of a promise is from an unfulfilled state. This is due to the fact that a promise is a proxy for an unknown value.

When the promise is filled with the value it's waiting for, it's in the fulfilled state. The promise will be labeled as failed if it returns an exception.

A promise may move from an unfulfilled to a fulfilled or failed state. Observers (or the objects/events waiting) are notified when the promise ...